<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button>按钮1</button>
  <button>按钮2</button>
  <button>按钮3</button>
  <button>按钮4</button>
  <button>按钮5</button>

  <script>
    var btns = document.querySelectorAll('button')

    for(var i = 0; i < btns.length; i++) {
      btns[i].onclick = function() {
        // 让所有人都先不红
        for(var j = 0; j < btns.length; j++) {  // 函数作用域
          btns[j].style.background = ''
        }

        // 让我自己变红
        this.style.background = 'red'
      }
    }
  </script>
</body>
</html>